luci-app-nlbwmon: new package

This commit introduces luci-app-nlbwmon, a frontend for nlbwmon, the
lightweight NetLink BandWidth Montor.

The nlbwmon daemon gathers per-host traffic statistics by querying netlink
accounting data. Due to this approach, the executable is very small and does
not rely on libpcap and CPU intensive raw sockets to monitor traffic.

Depends on PR https://github.com/openwrt/packages/pull/4646

Signed-off-by: Jo-Philipp Wich <jo@mein.io>